The present invention relates to low lead release plumbing components made of copper-based alloys containing lead, e.g lead brass components for potable water distribution circuits. The invention further relates to a method for obtaining the same by a selective surface etching thereof in order to either reduce or completely eliminate the labile surface Pb layer (almost exclusively consisting of Pb and/or Pb salts) responsible for the Pb release and representing the so-called Pb surface xe2x80x9csmearingxe2x80x9d.
Well known is the phenomenon leading to the creation of surface layers of metallic Pb (or of its salts), by segregation of Pb from the base alloy as a consequence of the thermal-mechanical stresses caused by machining and or molding of brass alloy elements containing lead. Such a phenomenon is a particularly undesirable one, in that the creation of the said lead surface layer may easily cause, at work, the release into the environment of Pb ions, a heavy metal known to be highly polluting and toxic to human health.
On the other hand, plumbing components such as mechanic parts for cocks and valves designed to operate in potable water distribution circuits and systems, cannot but undergo, during the manufacturing process, a number of machine work operations (lathing, drilling, threading, etc.). Moreover, a Cuxe2x80x94Zn base alloy containing also limited amounts of Pb (generally up to 3-5% by weight) facilitates machine working and leads to more effective and accurate surface finish. Furthermore, besides facilitating machine working (it furthers chip-breaking), the presence of Pb is also instrumental to the elements forming process, whether the latter is carried out directly by smelting or by molding/die-casting. Document DE-A4313439 solves the problem by isolating the inner surfaces of the component with a material free of Pb, which is difficult to apply.
The mechanism of Pb release has long been investigated and is based on the creation, on a zinc oxide surface layer, segregated from the base alloy, of Pb salts (hydroxycarbonates), due to surface stresses of the alloy as a consequence of both machining and shear stress during the molding process, and due further to Pb reactivity with water vapor and atmosphere carbon dioxide. It is however only very recently (March 1995) that a Certified testing procedure for evaluating the Pb release of plumbing components designed to potable water distribution has been approved and issued in print by the major United States Normalization Agency, i.e. N.S.F. The test procedure is known as U.S. NSF STD61. It has been shown that the phenomenon of Pb release is largely present in the commercial components for potable water distribution of any type, even in those components wherein surface coating, for example chromium or nickel plating, is extensively carried out, for haestetical reasons, on all the surfaces in view: in fact, the phenomenon depends on those limited surfaces designed to remain in contact with water when the taps, cocks ect. are closed, which are internal surfaces not in view and, therefore, normally not coated and, anyway, very difficult to be coated properly.
The aim of the present invention is therefore to furnish low lead release components made of copper-based alloys, in particular brass plumbing components for potable water distribution circuits, which, at the same time, can be subjected to usual working operations, by machining and/or molding, without any drawback with respect to the known alloys containing lead.
The present invention accordingly relates to mechanical components made of a copper-based alloy and and designed to be subjected, during their production stage, to working operations carried out either by machining, molding or die-casting, in particular plumbing components made of brass alloys and designed for potable water distribution systems, said components having respective surfaces defined by said alloy designed to be exposed, in use, to a fluid which is released in the environment, characterized in that said copper-based alloy contains a predetermined amount of lead as an alloying element; and in that, in combination, said surfaces of the components designed to be exposed to said fluid and defined by said alloy are free from surface enrichment of lead and lead salts.
In particular, said components are designed to collect potable water therein and are able to release in synthetic drinking water, after 15 days of test according to U.S. NSF STD61, an amount of Pb of no more than 2.5xc3x9710xe2x88x928 kg (0.025 xcexcg) for each liter (ml) of the internal volume of the components delimited by metallic surfaces exposed to contact with potable water during testing.
It is also included in the invention, according to a further aspect thereof, a mechanical component made of a copper-based alloy containing lead, and subjected, during its production stage, to working operations carried out either by machining, molding or die-casting, in particular a plumbing component made of brass and designed for potable water distribution systems, characterized in that respective surfaces of said component, which surfaces are designed to be contacted in use by potable water, present, under XPS surface analysis, an atomic surface composition such that the surface content in Pb is lower than or equal to the content in Pb according to the nominal composition of the alloy.
The invention further relates to a method for obtaining low Pb-release metal components made of copper-based alloys containing lead and designed to be employed in water distribution systems, in particular lead brass plumbing components for potable water circuits, said method comprising the following steps:
a selective etching of surfaces of said components designed to be exposed, at work, to the water, for removing almost entirely the Pb and Pb salts present thereon as a consequence of a mechanical working and/or of molding/die casting operations carried out onto said components; and
a passivation of said surfaces.
In particular, the selective etching step is carried out by exposing said surfaces to the action of a non-oxidizing acidic aqueous solution, of an acid capable of forming soluble Pb salts.
In particular, said acid is selected from the group consisting in: sulfamic acid, fluoboric acid, methanesulfonic acid, fluosilicic acid, acetic acid and mixtures thereof.
According to another embodiment of the invention, the selective etching step is carried out by exposing said surfaces to the action of an oxidizing acidic aqueous solution of an organic acid mixed with a peroxide. Preferably, the organic acid employed is citric acid and the peroxide is hydrogen peroxide.
Said passivation step follows said selective etching step and is carried out by exposure of said surfaces to the action of a basic aqueous solution, preferably a strong base aqueous solution.
Between said two steps, there is also provided for an intermediate rinsing stage.
Preferably, the basic aqueous solution contains a strong base selected from the group consisting in: NaOH, sodium silicate, and mixtures thereof and the passivation step is carried out keeping the solution to a pH comprised between 10 and 13.
Said exposure operations are carried out, according to the invention, by simply dipping said components into said treating solutions; while said rinsing operations are carried out by immersion in tap water at ambient temperature. Moreover, during said exposure to the action of said solutions, said solutions are subjected to ultrasonic agitation, in order to hit said surfaces of the components with ultrasonic waves.
In so doing, the ensuing selective etching of the surface lead, segregated from the alloy, affects, however, neither alloy composition nor surface finish resulting from machining (or from any other kind of working) to which said components have been subjected. Said etching operation, therefore, causes the surface lead, segregated from the alloy, to be removed so that lead is no longer released, during operation, by the elements so treated. Moreover, the removed lead can be easily recovered from the etchant, for example, by electrolysis, particularly in the presence of acid aqueous solutions. The afore process, therefore, guarantees high environmental safety.
The following passivating step, moreover, contributes to create on the exposed surfaces of said components an insoluble layer of corrosion chemicals which prevents both any possible corrosion process to be started in operation on the treated components, even in presence of aggressive fluids such as xe2x80x9csoft watersxe2x80x9d (potable waters having low contents of dissolved salts, especially of calcium), and the possible dissolution of the Pb not eliminated by the selective etching step (normally left inside open pores of the metallic matrix, which are deemed to be closed by the insoluble layers created by the passivation step.
Molarity range of the non-oxidizing acid, capable of forming soluble Pb salts, in the aqueous solution according to the invention, is 0.01-5 M and, in any case, its values are within the limits of the solubility scale of the chosen acid, while said solution has pH range 1-3. During immersion, according to the invention, the non-oxidizing acid etching solution is kept at a temperature ranging between 20xc2x0 C. and 50xc2x0 C. and immersion is carried out for 5 to 50 minutes.
According to the preferred embodiment, the machined elements, to be treated according to the invention, are degreased, rinsed, then dipped, for a period of time not exceeding 25 minutes, into a first aqueous solution of 0.1 M sulfamic acid, at 35xc2x0 C.-45xc2x0 C., then subjected to further rinsing, dipped into a second aqueous solution of 0.1 M sodium hydroxide, at 20xc2x0 C.-25xc2x0 C. and for a period of time not exceeding 15 min., and, finally, rinsed a third time and dried.
Rinsing is carried out in common tap water, at ambient temperature (13xc2x0 C.-20xc2x0 C.).
Finally, the preferred composition of the acidic aqueous solution is a mixture of 0.1 M sulfamic acid and 0.1 M fluoboric acid, in a 1:1 ratio, preferably added with a corrosion inhibitor.
According to a last aspect of the invention, therefore, it is provided an aqueous solution for performing a selective Pb etching mechanical components made of copper-based metal alloys containing Pb, the selective etching being directed against a surface enrichment in Pb and Pb salts of respective surfaces of said components which have been subjected to working operations carried out either by machining, molding or die-casting, said treating solution being characterized in having the following composition:
0.1 M sulfamic acid;
0.1 M fluoboric acid;
from 0.1 to 5% by weight of 1H-benzotriazole.
It is also included in the invention, a treating aqueous solution for performing the passivation of surfaces of mechanical components made of copper-based metal alloys containing Pb, said solution being characterized in containing, in combination: 0.1 M NaOH and from 1 to 5% by weight of sodium metaphosphite. The solution also includes sodium metasilicate, and/or a surface wetting agent, e.g. polyetoxyalchool.